Bipolar spin-transfer switching

ABSTRACT

Orthogonal spin-transfer magnetic random access memory (OST-MRAM) uses a spin-polarizing layer magnetized perpendicularly to the free layer to achieve large spin-transfer torques and ultra-fast energy efficient switching. OST-MRAM devices that incorporate a perpendicularly magnetized spin-polarizing layer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer, exhibit improved performance over prior art devices. The switching is bipolar, occurring for positive and negative polarity pulses, consistent with a precessional reversal mechanism, and requires an energy less than 450 fJ and may be reliably observed at room temperature with 0.7 V amplitude pulses of 500 ps duration.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority from U.S. Provisional Application61/414,724, filed Nov. 17, 2010, and is a continuation-in-part of U.S.application Ser. No. 13/041,104, filed Mar. 4, 2011, which, is adivisional of U.S. patent application Ser. No. 12/490,588, filed Jun.24, 2009, which is a continuation-in-part of U.S. patent applicationSer. No. 11/932,745, filed Oct. 31, 2007, which is acontinuation-in-part of U.S. patent application Ser. No. 11/498,303,filed Aug. 1, 2006, which is a continuation-in-part of U.S. patentapplication Ser. No. 11/250,791, filed Oct. 13, 2005, allowed Nov. 14,2006, and issued as U.S. Pat. No. 7,170,778 on Jan. 30, 2007, which is acontinuation of U.S. patent application Ser. No. 10/643,762, filed Aug.19, 2003, allowed Sep. 12, 2005, and issued as U.S. Pat. No. 6,980,469on Dec. 27, 2005, all of which are incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The present invention generally relates to magnetic devices such as usedfor memory and information processing. More particularly the inventiondescribes a spin-transfer torque magnetic random access memory(STT-MRAM) providing for bipolar spin-transfer switching.

BACKGROUND OF THE INVENTION

Magnetic devices that use a flow of spin-polarized electrons are ofinterest for magnetic memory and information processing applications.Such a device generally includes at least two ferromagnetic electrodesthat are separated by a non-magnetic material, such as a metal orinsulator. The thicknesses of the electrodes are typically in the rangeof 1 nm to 50 nm. If the non-magnetic material is a metal, then thistype of device is known as a giant magnetoresistance or spin-valvedevice. The resistance of the device depends on the relativemagnetization orientation of the magnetic electrodes, such as whetherthey are oriented parallel or anti-parallel (i.e., the magnetizationslie on parallel lines but point in opposite directions). One electrodetypically has its magnetization pinned, i.e., it has a higher coercivitythan the other electrode 50 and requires larger magnetic fields orspin-polarized currents to change the orientation of its magnetization.The second layer is known as the free electrode and its magnetizationdirection can be changed relative to the former. Information can bestored in the orientation of this second layer. For example, “1” or “0”can be represented by anti-parallel alignment of the layers and “0” or“1” by parallel alignment. The device resistance will be different forthese two states and thus the device resistance can be used todistinguish “1” from “0.” An important feature of such a device is thatit is a non-volatile memory, since the device maintains the informationeven tens of nanometers when the power is off, like a magnetic harddrive. The magnet electrodes can be sub-micron in lateral size and themagnetization direction can still be stable with respect to thermalfluctuations.

In conventional magnetic random access memory (MRAM) designs, magneticfields are used to switch the magnetization direction of the freeelectrode. These magnetic fields are produced using current carryingwires near the magnetic electrodes. The wires must be small incross-section because memory devices consist of dense arrays of MRAMcells. As the magnetic fields from the wires generate long-rangemagnetic fields (magnetic fields decay only as the inverse of thedistance from the center of the wire) there will be cross-talk betweenelements of the arrays, and one device will experience the magneticfields from the other devices. This cross-talk will limit the density ofthe memory and/or cause errors in memory operations. Further, themagnetic fields generated by such wires are limited to about 0.1 Teslaat the position of the electrodes, which leads to slow device operation.Importantly, conventional memory designs also use stochastic (random)processes or fluctuating fields to initiate the switching events, whichis inherently slow and unreliable (see, for example, R. H. Koch et al.,Phys. Rev. Lett. 84, 5419 (2000)).

In U.S. Pat. No. 5,695,864 and several other publications (e.g., J.Slonckewski, Journal of Magnetism and Magnetic Materials 159, LI(1996)), John Slonckewski described a mechanism by which aspin-polarized current can be used to directly change the magneticorientation of a magnetic electrode. In the proposed mechanism, the spinangular momentum of the flowing electrons interacts directly with thebackground magnetization of a magnetic region. The moving electronstransfer a portion of their spin-angular momentum to the backgroundmagnetization and produce a torque on the magnetization in this region.This torque can alter the direction of magnetization of this region andswitch its magnetization direction. Further, this interaction is local,since it only acts on regions through which the current flows. However,the proposed mechanism was purely theoretical.

Spin-transfer torque magnetic random access memory (STT-MRAM) deviceshold great promise as a universal memory. STT-MRAM is non-volatile, hasa small cell size, high endurance and may match the speed of static RAM(SRAM). A disadvantage of the common collinearly magnetized STT-MRAMdevices is that they often have long mean switching times and broadswitching time distributions. This is associated with the fact that thespin-torque is non-zero only when the layer magnetizations aremisaligned. Spin transfer switching thus requires an initialmisalignment of the switchable magnetic (free) layer, e.g. from athermal fluctuation. Relying on thermal fluctuations leads to incoherentreversal with an unpredictable incubation delay, which can be severalnanoseconds.

Spin-transfer torque magnetic random access memory (STT-MRAM) devicesuse current or voltage pulses to change the magnetic state of an elementto write information. In all STT-MRAM devices known to date,voltage/current pulses of both positive and negative polarities areneeded for device operation. For example, positive pulses are needed towrite a “1” and negative polarity pulses are needed to write a “0”. (Ofcourse, the definition of which magnetic state represents a “1” andwhich a “0” is arbitrary.) This magnetic element typically has twopossible states, magnetization oriented either “left” or “right”,parallel or antiparallel to the magnetization of a reference layer inthe device. These two magnetic states have different resistances, whichcan be used to read-out the information electrically.

Using present complementary metal-oxide-semiconductor (CMOS) technology,circuitry is needed to control the signals to STT-MRAM cells. PriorSTT-MRAM devices required bipolar sources and the bit cells were set toone state by one polarity and the other state by the other polarity,i.e. unipolar That is, the source needed to be able to provide bothpolarities because each polarity only could write either “0” or “1”.Although reading can be done with a unipolar voltage/current source,writing information required a bipolar source.

SUMMARY OF THE INVENTION

In view of the limitations associated with conventional designs ofdevices using spin transfer torque, an object of the present inventionis to provide a structure and methods that provide an improved magneticmemory or magnetic information processing device.

It is another object of the invention to produce a magnetic devicerequiring simplified external drive circuitry.

It is another object of the invention to produce a magnetic device thathas advantages in terms of speed of operation.

It is another object of the invention to produce a magnetic device thathas advantages in terms of reliability.

It is another object of the invention to produce a magnetic device thatconsumes less power.

These and other additional objects of the invention are accomplished bya device and methods that employ magnetic layers in which the layermagnetization directions do not lie along the same axis. For instance,in one embodiment, two magnetic regions have magnetizations that areorthogonal.

A further aspect of the invention provides a magnetic device that doesnot require a specific polarity of pulse. The magnetic device has atleast a first stable state and a second stable state. The application ofa pulse of appropriate amplitude and duration will switch the magneticdevice from whatever its current state is to the other state, i.e. fromthe first state to the second state or the second state to the firststate. Thus, the pulse source need only be unipolar and the bit cell isbipolar in that it can accept a pulse of either polarity.

BRIEF DESCRIPTION OF THE DRAWINGS

The forgoing and other features of the present invention will be morereadily apparent from the following detailed description and drawings ofthe illustrative embodiments of the invention wherein like referencenumbers refer to similar elements through the views and in which:

FIG. 1( a) illustrates a OST-MRAM layer stack, FIG. 1( b) is a graph ofdevice resistance vs. in-plane field showing 107% magnetoresistance (MR)and the switching of the free layer from the parallel (P) toantiparallel (AP) state at 12 mT and AP to P state at −16 mT. 1; FIG. 1(c) is a graph of device vibrating sample magnetometry (VSM) measurementsof the magnetization of the layer stack wherein the dotted line curveshows the switching of the free layer and (synthetic anti-ferramagnetic)(SAF) free layer under an in-plane applied field and the squared linered curve shows the characteristics of the polarizing layer, under afield perpendicular to the plane, demonstrating the high remanence and acoercive field of 50 mT;

FIG. 2 is an example of precessional switching and illustrates a pulsefor producing magnetization precession;

FIG. 3 is a graph of the switching probability from the P to the APstate as a function of pulse duration for three different pulseamplitudes at an applied field of 10 mT. 100% switching probability isachieved for pulses of less than 500 ps duration;

FIGS. 4( a) and 4(b) are graphs of switching probability as a functionof pulse amplitude at a fixed pulse duration of 700 ps with FIG. 4( a) Pto AP state; and FIG. 4( b) AP to P state wherein the switching isbipolar, occurring for both positive and negative pulse polarities;

FIG. 5 is an example of direct switching and shows a voltage trace of abit cell switching event;

FIGS. 6( a)-6(f) illustrate the statistical probability from P to AP asa function of the pulse amplitude; larger pulse amplitudes produceshorter switching start times and shorter times to switch;

FIGS. 7( a)-7(c) illustrate typical device characteristics for a 50nm×115 nm ellipse shaped bit cell; the resistance is measured as afunction of applied in-plane magnetic field; FIG. 7( a) shows theapplied field induced switching of the reference and free layers; FIG.7( b) shows the applied field induced switching of just the referencelayer and FIG. 7( c) shows the applied field induced switching of justthe free layer;

FIGS. 8( a)-8(c) show for conditions (β=1, a_(J)=+0.025) themagnetization switching is precessional, starting at time zero from a Pstate; the three components of the magnetization are show, m_(x), m_(y)and m_(z);

FIGS. 9( a)-9(c) show for conditions (β=1, a_(J)=−0.025) themagnetization switching is precessional, this shows that both positiveand negative polarity pulses lead to precessional magnetizationreversal, with somewhat different rates (or frequencies) of precession;

FIG. 10( a)-10(c) show for conditions (β=5, a_(J)=+0.008) themagnetization switching from P to AP is direct (i.e. there is noprecession);

FIGS. 11( a)-11(c) show for conditions (β=5, a_(J)=−0.008) there is noswitching from the P state; only a positive pulse (FIG. 10( a)-10(c))leads to magnetization switching from the P to the AP state;

FIGS. 12( a)-12(c) show for conditions (β=5, a_(J)=−0.006, i.e. negativepulse polarities) there is direct switching from the AP to P state; and

FIGS. 13( a)-13(c) show for conditions (β=5, a_(J)=+0.006, i.e. positivepulse polarities) there is no switching from the AP to P state,switching from the AP to P state only occurs for negative pulsepolarities (FIG. 12( a)-12(c)).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to orthogonal spin transfer MRAM(OST-MRAM) devices and methods. OST-MRAM employs a spin-polarizing layermagnetized perpendicularly to a free layer to achieve large initialspin-transfer torques. This geometry has significant advantages overcollinear magnetized STT-MRAM devices as it eliminates the nanosecondincubation delay and reduces the stochastic nature of the switching. Italso has the potential for write times below 50 ps. FIG. 1( a)illustrates one embodiment of a STT-MRAM. A perpendicularly magnetizedpolarizer (P) is separated by a non-magnetic metal from the freemagnetic layer (FL). The free layer forms one electrode of a MTJ. Theother electrode, the reference layer, consists of an SAF free layer.

In OST-MRAM the reference magnetic layer is used to read out themagnetic state. The magnetization of this layer is set to be collinearto that of the free layer and the memory states correspond to the freelayer magnetization parallel (P) or antiparallel (AP) to the referencelayer magnetization. Previous OST-MRAM devices utilized an out-of-planemagnetized spin-polarizer was combined with an in-plane magnetizedspin-valve. While fast switching was seen, the resulting read-outsignals were small; there was less than ˜5% magnetoresistance (MR). Thedevice impedance was also small, ˜5 Ω.

One embodiment of the present invention is directed to a magnetic tunneljunction (MTJ) based OST-MRAM device that combines fast switching andlarge (>100%) MR, both of which are critical for applications. Thedevice impedance is ˜1 kΩ and thus compatible with complementarymetal-oxide-semiconductor (CMOS) field effect transistor (FET) memorycontrol circuitry. The write function switches the state of the cell,rather than setting the state. Further, the switching is bipolar,occurring for positive and negative polarity pulses, consistent with aprecessional reversal mechanism.

In one embodiment, the present invention provides apparatus and methodsenabling a “toggle” mode of spin-transfer device operation. The pulsesource may be unipolar because the bit cell does not set the state basedupon the polarity of the pulse, i.e. it is bipolar. Rather a pulse ofsufficient amplitude for either polarity will “toggle” the magneticstate of the device, “1”→“0”and “0”→“1”. Thus, a pulse (of sufficienttime and amplitude) will change the magnetic state of the device or bitcell, irrespective of the original magnetic state. As such, in oneembodiment, the writing of information in such a “toggle” mode ofoperation may require reading the device or bit cell initial state andeither applying or not applying a current/voltage pulse depending on theinformation to be written. That is, if the device or bit cell wasalready in the desired state no pulse would be applied.

As an example of this embodiment, FIG. 2 illustrates an experimentaltime resolved voltage trace of a bit cell switching event. A voltagepulse of −0.62 V is applied for about 2 ns, starting at time zero in theplot. The device is a 50 nm×115 nm ellipse shaped bit cell and has animpedance of about 2 kOhm. The horizontal dashed trace at the signallevel of 1 corresponds to the antiparallel state (AP). The horizontaldashed trace at signal level 0 shows the response when the bit cell isin the parallel (P) state. (P and AP refer to the magnetizationdirection of the free layer with respect to the reference layermagnetization in the stack.) At about 1.1 ns the device switches fromthe P to the AP state. The device then precesses at a frequency of about3 GHz. The device final state (P or AP) depends in the pulse duration.

The use of a bipolar toggle for STT-MRAM allows the external drivecircuitry to be simplified because all device operations (i.e., readingand writing) can be accomplished with a power source of one polarity. Inaddition, it is believed that devices utilizing the present inventionwill likely operate faster because pulses of less than 500 psec willtoggle the magnetic state of the device. An additional benefit of theneed for only one polarity is that power consumption will be reduced.This is due, in part, to the fact that the supply voltages to the devicedo not need to be switched between different levels. In presentMRAM-CMOS designs, typically one transistor is associated with each MRAMbit cell and the source and drain voltages on this transistor need to bevaried to write the information. In accordance with one embodiment ofthe present invention, the source or drain voltages may be maintained atconstant levels. Maintaining the source or drain voltages at a constantlevel(s) reduces the power required for device operation, as each timethe polarity of a supply voltage is changed energy is required. In oneembodiment, switching requires an energy of less than 450 fJ in a freemagnetic layer that is thermally stable at room temperature.

As previously mentioned hereinbefore, one characteristic of thedescribed OST-MRAM devices is that the switching is bipolar, i.e. a bitcell in accordance with the present invention may be switched betweenstates using either voltage pulse polarity. However, there can bethresholds for the pulse to trigger a switch. Those thresholds maydiffer depending on the pulse, for example depending on the pulsepolarity or depending on the device initial state, i.e. P or AP. Thischaracteristic is illustrated further below regarding the examples inFIGS. 4( a) and 4(b).

This asymmetry in the probability distribution for the two polarities isdistinct from the characteristics seen in common collinear freelayer/tunnel barrier/SAF type STT devices. In these devices switchingonly occurs for one polarity of the voltage pulse, or through thermallyinduced backhopping. As previously stated, in OST-MRAM devices switchingoccurs for both polarities. This bipolar switching process is anindication that the torque originates from the perpendicular polarizer.For a collinear device we would expect P→AP switching only for positivepolarity pulses, based on spin-transfer torque models. For oneembodiment of the invention, a positive polarity pulse leads to a lowerswitching probability (FIG. 4( a)) compared to the opposite polaritypulse. If the switching processes involved simple heating of thejunction, rather than the spin-transfer torque switching in the OST-MRAMdevice as described herein, it would be expected that a symmetricswitching probability distribution and a monotonic dependence of theswitching probability on pulse amplitude would be observed, which is notthe case as seen in FIGS. 4( a) and 4(b).

As an example of this embodiment, FIG. 5 illustrates an experimentaltime resolved voltage trace of a bit cell switching event. A voltagepulse of 0.7 V is applied for about 2 ns, starting at time zero in theplot. The device is a 60×180 nm² shaped hexagon and has an impedance oforder of 1 kOhm. The dotted line trace “a” shows the response when thebit cell is in the parallel state (P). The line trace “b” shows theresponse when the bit cell is in the antiparallel (AP) state. (P and APrefer the magnetization direction of the free layer with respect to thereference layer magnetization in the stack.) The line trace “c” shows anevent in which the device switches from P to AP about 1.2 ns after thestart of the pulse. The line trace “d” shows the same data filtered toremove the high frequency components, which is associated with noise.The start time and switching time are defined as indicated in this FIG.5.

The free layer magnetization rotation about its demagnetizing field willresult in a switching probability that is a nonmonotonic function of thepulse amplitude or duration, because if the pulse ends after the freelayer magnetization finishes a full rotation (i.e., a 2 π rotation), theprobability of switching will be reduced. The precession frequency canalso be a function of the pulse polarity due to the fringe fields fromthe polarizing and reference layers. Further, spin-torques from thereference layer break the symmetry of the reversal by adding torquesthat favor one free layer state over another.

Thus, a device in accordance with the principles of the presentinvention can utilize voltage/current pulses of just one polarity towrite both “0” and “1” states. A device initially in the “1” state canbe switched to a “0” state and a device originally in the state “0” canbe switched into the state “1” with the same polarity pulse. Further,although the pulse amplitudes needed for these operations can differ(see FIGS. 4( a) and 4(b)), this aspect can be used to advantage indevice operation. For example, if the thresholds differ then the pulseamplitude can uniquely determine the device final state. In oneembodiment, the differences between the thresholds for a positivepolarity pulse and a negative polarity pulse can be significant enoughthat a read step would be unnecessary. For example, where a negativepulse requires a much lower amplitude and or pulse duration to achieve a100% probability of a switch from P to AP than the positive pulse andvice versa, the positive pulse requires a much lower amplitude or pulseduration to achieve 100% probability of a switch from AP to P, thosethresholds can be utilized to achieve a desired functionality. If thepositive pulse that is necessary to switch from AP to P would be belowthe threshold necessary to switch from P to AP and vice versa for thenegative pulse, then a device need not read the bit cell prior to awrite. For example, where a write of the bit cell to P state is desired,the device can be pulsed with a sufficient (above the 100% threshold forswitching to P but below that threshold for a switch to AP) positivepulse. If the bit cell is in AP, the positive pulse is sufficient toswitch to P. However, if the bit cell is already in P, the positivepulse would be insufficient (i.e. below the threshold) to switch to AP.

The following non-limiting Examples illustrate various attributes of theinvention.

Example 1

The OST-MRAM layer stack was grown on 150 mm oxidized silicon wafersusing a Singulus TIMARIS PVD module. The device layer structure isillustrated in FIG. 1( a). The polarizer consists of a Co/Pd multilayerexchange coupled to a Co/Ni multilayer. The Co/Ni multilayer has a highspin polarization due to the strong spin-scattering asymmetry of Co inNi and a perpendicular magnetic anisotropy (PMA). To enhance the layercoercivity and remanence this layer is coupled to Co/Pd which has a verylarge PMA but a lower spin polarization due to the strong spin-orbitscattering by Pd. The polarizer is separated by 10 nm of Cu from anin-plane magnetized CoFeB free layer that is one of the electrodes of aMTJ. The MTJ structure is 3 CoFeB|0.8 MgO|2.3 Co_(0.4)Fe_(0.4)B_(0.2)|0.6 Ru|2 Co_(0.4) Fe_(0.3) 16 PtMn (number to the leftof each composition indicates the layer thicknesses in nm). The waferwas annealed at 300° C. for 2 hours in a magnetic field and thencharacterized by vibrating sample magnetometry (VSM), ferromagneticresonance spectroscopy (FMR), and current-in-plane-tunneling (CIPT)measurements. FIG. 1( c) shows VSM measurements of the filmmagnetization for in-plane and perpendicular-to-the-plane appliedfields. The free layer is very soft while the reference layer has acoercive field of about 50 mT; the exchange bias from theantiferromagnetic PtMn is 100 mT. The perpendicular polarizer has acoercive field of 50 mT.

The wafers were patterned to create OST-MRAM devices using e-beam andoptical lithography. Ion-milling was used to etch sub-100 nm featuresthrough the free layer. Device sizes varied from 40 nm×80 nm to 80nm×240 nm in the form of rectangles, ellipses and hexagons.Approximately 100 junctions were studied. Set forth in greater detailbelow are results obtained on one 60 nm×180 nm hexagon shaped device.Although not presented here, similar results have been obtained on otherdevices of this type.

The sample resistance was measured by applying a small voltage (V_(dc),=30 mV) and measuring the current. The MR of the device is mainlydetermined by the relative orientation of the free (3 CoFeB) andreference (2.3 CoFeB) layers, which can be either parallel (PA) orantiparallel (AP). FIG. 1( b) shows the minor hysteresis loop of thefree layer. The patterned free layer has a coercive field of 14 mT atroom temperature and the device has 107% MR. The loop is centered atabout −2 mT, due to a small residual dipolar coupling from the syntheticantiferromagnetic (SAF) reference layer.

To measure the current-induced switching probability, pulses of variableamplitude, duration and polarity were applied. An applied field was usedto set the sample into the bistable region (see FIG. 1( b)) and thenvoltage pulses were applied, using a pulse generator that provides up to2 V amplitude with a minimum pulse duration of 50 ps. By measuring theresistance using a small dc voltage before and after the pulse, we candetermine the device state. Since the free layer is very stable withoutany applied voltage (see the discussion below), it can be assumed that aswitching event (i.e., dynamics of the free layer magnetization to apoint at which the free layer magnetization would reverse in the absenceof the pulse) occurred during the voltage pulse. In this setup, positivevoltage is defined to correspond to electrons flowing from the bottom tothe top of the layer stack, i.e. from the polarizer toward the free andreference layers.

Both amplitude and pulse duration where observed to impact theprobability of a switching event. FIG. 3 shows the switching probabilityfrom the P to the AP state as a function of pulse duration in an appliedfield of 10 mT for three different pulse amplitudes, −0.5, −0.6 and −0.7V. Higher amplitude pulses lead to switching at shorter pulse durations.It has been observed that a device in accordance with the presentinvention can be switched with pulses less than 500 ps in duration with100% probability. The energy needed for 100% probability switching isless than 450 fJ. As 100% switching probability was observed for pulsesas short as 500 ps, it is believed that there is no incubation delay ofseveral nanoseconds as observed in conventional collinear or nearlycollinearly magnetized devices. The switching process of the presentinvention thus provides both fast and predictable results.

To determine the energy barrier of the reversal, the coercive field ofthe sample is measured at different field sweep rates. The energybarrier is then determined from the relation:

$\begin{matrix}{{\tau = {\tau_{0}{\exp \left\lbrack {\xi_{0}\left( {1 - \frac{H_{app}}{H_{c}}} \right)}^{\beta} \right\rbrack}}},} & (1)\end{matrix}$

where ξ₀=U₀/kT, the zero applied field energy barrier over the thermalenergy, with T=300 K. Assuming β=2, we obtain an energy barrier of ξ=40at μ₀H_(app)=0.01 T, indicating the layer is very thermally stable atroom temperature.

As previously mentioned, one characteristic of the described OST-MRAMdevices is that the switching is bipolar, i.e. a bit cell in accordancewith the present invention may be switched between states using eithervoltage pulse polarity. For the examples described above, FIG. 4( a)shows the switching probability versus pulse amplitude for (a) P→APswitching and FIG. 4(b) shows AP→P switching at a pulse duration of 700ps. Although the OST-MRAM device is bipolar, it can be seen in FIG. 4(a) that negative polarity pulses lead to higher switching probabilitythan positive polarity pulses. The opposite is found in FIG. 4( b) forAP→P. In both cases applied fields closer to the coercive field lead toa lower voltage pulse threshold for switching. Also the switchingprobability is a nonmonotonic function of the pulse amplitude. Theobserved data is qualitatively consistent with precessional reversalbeing driven by the perpendicular polarizer.

Example 2

The magnetization dynamics of the device and method in the preferredembodiment can be modeled to a first approximation by considering thespin transfer torques associated with the perpendicular

$\begin{matrix}{\frac{\hat{m}}{t} = {{{- \gamma}\; \mu_{0}\hat{m} \times {\overset{->}{H}}_{eff}} + {\alpha \; \hat{m} \times \frac{\hat{m}}{t}} + {\gamma \; a_{J}\hat{m} \times \left( {\hat{m} \times {\hat{m}}_{P}} \right)} - {\beta \; \gamma \; a_{J}\hat{m} \times \left( {\hat{m} \times {\hat{m}}_{R}} \right)}}} & (2)\end{matrix}$

polarizer and the reference layer as follows:

where m represents the magnetization direction of the free layer (it isa unit vector in the direction of the free layer magnetization). α isthe damping parameter of the free layer. The prefactor, a_(J), dependson the current density J, the spin-polarization P of the current densityJ, and the cosine of the angle between the free and pinned magneticlayers, cos(θ), such that a_(J)=hJg(P,cos(θ))/(eMt). The h is thereduced Planck's constant, g is a function of the spin-polarization Pand cos(θ), M is the magnetization density of the free layer, e is thecharge of an electron, and t is the thickness of the free layer. Thelast two terms are the spin transfer from the perpendicular polarizer(m_(P)) and the in-plane magnetized reference layer (m_(R)). The β(beta) represents the ratio of the magnitude of these two torques.

Analysis of this equation shows that the ratio β (beta) is important incontrolling the magnetization dynamics. Higher β (greater than 1)results in a range of current pulse amplitudes in which the switching isdirectly from P to AP for one current polarity and AP to P for the othercurrent polarity. For higher current amplitudes the switching isprecessional (toggling from AP to P to AP and continuing, as shownexperimentally in FIG. 2). The switching is bipolar in this case,occurring for both current polarities. The device impedence is about 2-4k Ohms as shown in FIGS. 7( a)-7(c).

For small beta (beta less than or about equal to 1) the range of currentpulse amplitudes in which direct switching occurs is reduced. The motionfor small β becomes precessional (toggling from AP to P to AP andcontinuing, as was seen in experiments shown in FIG. 2). When themagnetization motion is precessional higher current amplitudes generallyresult in higher precession frequencies.

The presence of the polarizer (in all cases cited above) reduces thetime needed to set the bit cell state (see FIGS. 6( a)-6(f)) improvingdevice performance, both reducing the switching time and reducing thecurrent (or voltage) amplitude needed for switching. Calculations of theswitching dynamics based on model described in Eqn. (2) above show thetype of characteristics that can be found in OST-MRAM stacks. Thebehavior was determined for a thin film nanomagnet with in-planeanisotropy field (along x) of 0.05 T, damping (α=0.01), magnetizationdensity of μM_(s)=0.5T and the magnetization of the reference layer inthe +x direction.

Example 3

In certain embodiments the reliable writing in which pulse duration isnot critical. In such a case, for a memory operation, it may bepreferable that the pulse duration not be a critical variable (i.e. theprecise pulse duration would not determine the bit cell final state;only the pulse polarity—positive or negative—would be important. In thiscase, the device is provided with β about equal to or greater than 1.This can be accomplished in a number of ways:

The spin-polarization from the reference layer can be increased bychoice of materials for the magnetic tunnel junction and referencelayer. For example, CoFeB in contact with MgO has a largespin-polarization. Permalloy (NiFe) in contact with MgO has a lower spinpolarization.

The spin-polarization from the polarizing layer can be reduced. This canbe accomplished in a number of ways, for example, without limitation:

-   -   a. Choice of materials for the composition of the polarizing        layer: Co/Ni multilayers have a large spin-polarization.        However, Co/Pd or Co/Pt have a much lower spin-polarization. A        composite polarizing layer can have an adjustable polarization.        For example a multilayer of Co/Ni on Co/Pd or Co/Pt in which the        thickness of the Co/Ni is varied (from, say 0.5 to 5 nm) is a        means to control the current spin-polarization from the        polarizing layer, where the Co/Ni is the layer in closer        proximity to the free magnetic layer. A thin layer with large        spin-orbit coupling, such as Pt or Pd, could also be placed on        the surface of the polarizing layer closer to the free magnetic        layer. This would also serve to reduce the current        spin-polarization.    -   b. Alternatively, the nonmagnetic layer between the polarizer        and the free layer can be varied to control the        spin-polarization of carriers from the polarizing layer (and        thus the parameter β). If this layer has a short spin-diffusion        length the polarization would be reduced. Including defects in        Cu can reduce its spin-polarization (e.g., Ni in Cu or other        elements). The Cu can also be an alloy with another element,        such as Zn or Ge. There are many possible material combinations        that would reduce the spin-polarization from the polarizing        layer.

For fast low energy switching it would be preferable to not increase βfar beyond 10, because the torque from the perpendicular polarizer setsthe switching time and thus the energy required to switch the device (asdiscussed above).

For fastest write operation: β preferably should be less than one, andthe pulse timing needs to a very well-controlled variable. The switchingis bipolar and only a single polarity voltage source is needed fordevice write operations, potentially simplifying the drive circuitry.

Analysis of the model described above (see section [0043]) also showsthat the threshold voltage and current for switching can be reducedthrough a number of means. First, the free layer magnetization densityor free layer thickness can be reduced. However, this also is expectedto reduce the bit cell stability. So the magnetization density or freelayer thickness cannot be made arbitrarily small. Second, the free layercan have a component of perpendicular magnetic anisotropy. Thisanisotropy would be insufficient in and of itself to reorient themagnetization perpendicular to the layer plane, but would nonetheless beeffective in reducing the switching current and voltage. This kind ofperpendicular anisotropy is seen in thin (0.5 to 3 nm thick) CoFeBlayers in contact with MgO. The switching speed and free layerprecession frequency depends on the free layer perpendicular anisotropy.Larger perpendicular anisotropy leads to lower frequency precession,reducing the switching speed. Third, the damping parameter of the freelayer can be reduced to reduce the switching current and voltage. Thisand other means may be used to reduce the switching voltage and currentthreshold.

The foregoing description of embodiments of the present invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the present invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of thepresent invention. The embodiments were chosen and described in order toexplain the principles of the present invention and its practicalapplication to enable one skilled in the art to utilize the presentinvention in various embodiments, and with various modifications, as aresuited to the particular use contemplated.

1. A magnetic device comprising: a perpendicularly magnetized polarizinglayer; a free magnetic layer, the free magnetic layer forming a firstelectrode and separated from the magnetized polarizing layer by a firstnon-magnetic metal layer, the free magnetic layer having a magnetizationvector having at least a first stable state and a second stable state; areference layer forming a second electrode and separated from thefree-magnetic layer by a second non-magnetic layer; wherein applicationof a current pulse, having either positive or negative polarity and aselected amplitude and duration, through a magnetic device switches themagnetization vector.
 2. The magnetic device as defined in claim 1wherein spin transfer torques associated with the perpendicularlymagnetized polarized layer and an in-plane magnetized form of thereference layer can be described by,$\frac{\hat{m}}{t} = {{{- \gamma}\; \mu_{0}\hat{m} \times {\overset{->}{H}}_{eff}} + {\alpha \; \hat{m} \times \frac{\hat{m}}{t}} + {\gamma \; a_{J}\hat{m} \times \left( {\hat{m} \times {\hat{m}}_{P}} \right)} - {\beta \; \gamma \; a_{J}\hat{m} \times \left( {\hat{m} \times {\hat{m}}_{R}} \right)}}$where m represents a magnetization direction of the free layermagnetization, a_(j) is a term proportional to current of the currentpulse and current spin-polarization, the third term on the right handside of this equation being spin transfer torque from a perpendicularpolarizer (m_(P)) and the fourth term on the right hand side of theequation being a spin transfer torque from the in-plane magnetized formof the reference layer (m_(R)), and β represents a ratio of magnitude ofthese spin transfer torques.
 3. The magnetic device as defined in claim2 wherein β>1 provides a range of current pulse amplitudes whereinswitching of the magnetic device is directly from parallel toanti-parallel for a first current polarity and anti-parallel to parallelfor a second current polarity.
 4. The magnetic device as defined inclaim 2 wherein the device switching is precessional and bipolar forboth polarities for selected values of β.
 5. The magnetic device asdefined in claim 2 wherein beta less than or about equal to 1 providesat least one of reduced direct current switching errors and precessionaland fast switching.
 6. The magnetic device as defined in claim 2 whereinthe magnetization direction becomes precessional for the beta less thanor about equal to 1, thereby providing higher precession frequencies forhigher current amplitudes.
 7. The magnetic device as defined in claim 2wherein β is selected from the group of about 1 or greater than 1 andpulse polarity controls a final magnetization state of the free magneticlayer and independent of the current pulse duration.
 8. The magneticdevice as defined in claim 7 wherein spin polarization of the referencelayer is increased by selecting mating materials for the reference layerand a magnetic tunnel junction layer adjacent thereto.
 9. The magneticdevice as defined in claim 8 wherein the mating materials are selectedfrom the group of (a) CoFeB and MgO, (b) NiFe and MgO, and (c) CoFe andMgO.
 10. The magnetic device as defined in claim 7 wherein spinpolarization from the polarizing layer is reduced by selecting aparticular composition therefore.
 11. The magnetic device as defined inclaim 10 wherein the particular composition comprises a Co/Nimultilayer.
 12. The magnetic device as defined in claim 11 wherein themultilayer is selected from the group of Co/Ni on Co/Pd, CoNi on Co/Pt.13. The magnetic device as defined in claim 12 wherein thickness of theCo/Ni can be varied, thereby controlling the spin polarization.
 14. Themagnetic device as defined in claim 7 wherein spin polarization from thepolarizing layer is reduced by further including a nonmagnetic layerdisposed between the polarizing layer and the free magnetic layer,thereby controlling the spin polarization of carriers from thepolarizing layer.
 15. The magnetic device as defined in claim 14 whereinthe nonmagnetic layer comprises Cu with controlled defects, therebyreducing the spin polarization.
 16. The magnetic device as defined inclaim 14 wherein the nonmagnetic layer can have varying layer thickness,thereby reducing spin polarization from the polarizing layer incident onthe free magnetic layer.
 17. A method of controlling a memory arrayhaving a plurality of cells each comprising: determining an initialstate of a cell within the memory array determining if the initial stateis the same as a write state corresponding to information to be writtento the cell; if the initial state is different from the write state,applying a current pulse of either positive or negative polarity and ofa selected amplitude and duration through the magnetic device switchingthe magnetization vector.
 18. A memory array comprising: at least onebit cell including: a magnetic device having: a magnetic layer having afixed magnetization vector; a free magnetic layer having a variablemagnetization vector having at least a first stable state and a secondstable state; a non-magnetic layer separating the magnetic layer withfixed magnetization vector and the free magnetic layer; whereinapplication of a current pulse having either positive or negativepolarity and a sufficient amplitude and duration through the magneticdevice switches the magnetization vector from either the first stablestate to the second stable state or from the second stable state to thefirst stable state; and at least one transistor for current control andreadout wherein application of a voltage to the at least one bit cellactivates the bit cell.